The most common oxidizer used in solid rocket propellants is ammonium perchlorate (“AP”), NH4ClO4. This oxidizer is used because of its high oxygen content and large gas volume that is generated during combustion. However, the chlorine in the oxidant generally favors formation of hydrogen chloride as its primary product species. In common propellant formulations using AP, as much as 98% of the available chloride ion may be converted to hydrogen chloride. Hydrogen chloride pollution has serious negative consequences both to the environment and the ozone layer and is a major contributor to launch site equipment corrosion. Hydrogen chloride also forms nucleation sites for aerosolized water products which contributes to secondary smoke formation from exhaust plumes thereby making rockets easier to detect, which is a disadvantage for some applications.
There are three methods that have been investigated generally for hydrogen chloride reduction in composite rocket propellants, namely using reduced chlorine propellants, neutralizing hydrogen chloride, and the use of scavengers.
Using reduced or non-chlorine containing oxidizers reduces or eliminates hydrogen chloride formation, though generally at an unacceptable loss in performance or increase in detonation sensitivity. Ammonium nitrate (“AN”) has been widely investigated as an AP replacement, but its performance has been shown to be much lower than AP and with less ballistic tailorability. Others have investigated incorporating nitrate esters and/or nitroamines in place of all or part of the AP oxidizer. While these energetic formulations have been widely used for their high performance, they are often more sensitive and are considered Class 1.1 propellants which make their handling substantially more dangerous.
Acid-base chemistry has been investigated as a way to neutralize hydrogen chloride formed from solid rocket propellants by using magnesium as a fuel in propellants. When burned with AP, magnesium oxide and hydrogen gas are formed. Hydrogen gas will subsequently after burn with ambient oxygen outside of the rocket motor to form water which in turn reacts with the magnesium oxide to form magnesium hydroxide. The magnesium hydroxide may then react with hydrogen chloride to form magnesium chloride and water. While this propellant has adequate theoretical performance and neutralizing effects, it also has a low density thus requiring a heavier loading and a serious disadvantage in that it requires sufficient atmospheric oxygen to combust hydrogen. Due to the low levels of oxygen at higher altitudes, this approach has limited applicability.
Another possible method for reducing hydrogen chloride formation would be to use strongly halophilic materials, such as alkali metals, to “scavenge” chlorine ions during combustion to form alkali-metal chlorides. Because of the high reactivity of these materials and potential health effects, alkali metals are generally not used as neat elemental metals in formulations. Therefore, alkali metal nitrates, such as LiNO3 and NaNO3 have been used to stably introduce the halophilic material, replacing a stoichiometric amount of AP. While these formulations do reduce the hydrogen chloride formation during combustion, they do so at an unacceptable loss to performance. It has been shown that the addition of nitroamines to the formulation can offset this performance deficit to some degree, but their addition result is a Class 1.1 propellant, which is extremely dangerous to handle.
While some general research into dual oxophilic-halophilic fuel combinations has been investigated as a means to improve specific impulse (ISP) when combined with a chlorine-containing oxidant, such as disclosed in U.S. Pat. No. 3,133,841, such use as hydrogen chloride scavengers has previously not been investigated and the range of alloys presented lack suitable performance characteristics. There is, therefore, an unmet need for a high performance solid-fuel rocket propellant capable of significantly reducing hydrogen chloride formation.